1. Field of the Invention
The present invention relates generally to a shock absorber employable for a back rest of a chair. More particularly, the present invention relates to a shock absorber operable for properly absorbing shock caused when an user sits on the chair and then reclines against the back rest of the chair.
2. Description of the Related Art
A chair of the type including a pair of turnable support arms for supporting a reclinable back rest at the upper ends thereof has been hitherto known as disclosed in official gazettes of, e.g., Japanese Patent Laid-Open Pub. NO. 63-186605 and Japanese Utility Model Laid-Open Pub. NO. 64-32658.
According to each of the prior inventions, a quantity of expansion/contraction of compression springs for absorbing shock caused by inclination of the back rest in the rearward direction is adequately adjusted by rotating a handle with an user's hand so as to vary an intensity of biasing force generated by the compression springs, whereby an optimum intensity of shock absorbing function is provided for the back rest of the chair.
With the conventional chair constructed in the above-described manner, since an intensity of repulsive force from the compression springs is increased as the compression springs are compressed more and more by rotating the handle to enhance the biasing force generated by the compression springs, there arises a problem that the handle should be rotated with a higher intensity of force as the biasing force is increased, resulting in the handle being rotated with much difficulties.
When an intensity of the biasing force is variably adjusted, the compression springs are compressed or expanded within the allowable extent of compression and expansion of the compression springs. However, since the intensity of the biasing force largely varies merely by slight rotation of the handle, there arises other problem, that it is difficult to finely adjust the biasing force within the wide range of intensity.
Once the biasing force of the compression springs is adjusted to assume a high intensity, a strong shock absorbing function appears immediately after an user reclines against the back rest of the chair to enjoy a pleasant resting attitude. For this reason, there arises another problem that an adequate shock absorbing function can not be obtained with the conventional chair when the user reclines against the back rest of the chair.